Lasers are commonly used micro- and nano-machining of a wide range of materials, such as the drilling, dicing, trimming or milling of various metals, ceramics, polymers and other materials, including the drilling, dicing and milling of printed circuit boards, substrates and integrated circuits. Such laser machining processes employ a variety of lasers to generate the machining beams, such as gas and solid state lasers, and have recently begun to use fiber optic lasers, also referred to as fiber lasers Fiber lasers offer a number of advantages in the physical construction and configuration of a laser machining system, but must typically be driven, or pumped by other lasers, such as solid state diode lasers. Fiber lasers also suffer from other problems, which are often shared with or stem from the lasers used to pump the fiber lasers.
Considering the problems arising from the lasers used to pump the fiber lasers, the Diode Pumped Solid State (DPSS) lasers currently and commonly used to drive fiber lasers produce a single mode beam and typically suffer from pointing and drift instabilities as well as from limits on the power that can be delivered by the DPSS laser, which is typically in the range of 10 Watts @ 355 nm (nanometers) and 2 to 3 watts @266 nm.
That is, and for example, DPSS lasers may produce useful Gaussian and TEM00 mode beam outputs as well as a single mode beam outputs, but these modes, and in particular the single mode beam output, do not provide the highest output power. DPSS lasers thereby require the use of beam shaping technology to produce a uniform flat top beam that is useful for driving a fiber laser system. Beam shaping technology and uniform flat top beams, however, require a stable laser input beam and, discussed below, it is difficult to obtain a stable output beam from a DPSS laser.
For example, the DPSS lasers commonly used for drilling multi-layered printed circuit boards for producing chip packaging/carriers generate a single mode output beam that becomes unstable when certain critical parameters, such as pump current and pulse repetition rate, are changed, or because of certain conditions arising from operation of the laser, such as thermal lensing or heating due to power dissipation. Such variations in the laser parameters may result, for example, in long term beam movement or drift and beam drift and pointing instability drift. Long term beam movement or drift may result in system down time since the optical beam delivery system must be realigned when the beam displacement exceeds tolerable limits, while short term beam movement or drift will typically result in distortion of the beam profile, the formation of hot and cold spots in the beam, and a loss in efficiency and in the power levels delivered by the laser beam.
Certain other problems are common and critical to fiber laser systems as well as to the laser machining systems of the prior art and one of the most significant of these problems is control of the laser beam pulse width, repetition rate, frequency and power characteristics to achieve the optimum effect at the workpiece being machined.
To illustrate, the laser machining systems of the prior art that employ UV range DPSS lasers to form microvias in multi-layered materials rely upon an ablation process to drill the microvias. The performance of DPSS UV lasers is limited in its processing capabilities, however, due to its beam propagation factor, which is typically in the range M2=<1.2, as well as by the fact that the drilling process is limited solely to UV ablation. UV DPSS lasers are also limited in their ability to dice or cut multilayered printed circuit board materials and other semiconductor wafer materials due to the etch rate of materials in of UV wavelengths, which is a function of the absorption of UV light in the materials, and because of variations or differences in the ablation energy density required for different materials as the DPSS laser cuts through the single and multiple layers of different materials. For example, the ablation of polymers requires relatively low energy densities while metals and ceramics require higher energy densities while DPSS lasers in the UV range have a relatively limited etch rate range of approximately 0.3 to 0.5 microns/pulse, depending on the energy density of the laser.
It must also be noted that while the etch rate of a given laser can be increased by increasing the laser output power, repetition rate or pulse width, or any combination thereof, such efforts can as easily be counterproductive for a number of reasons. For example, increases in one of the output beam power, the repetition rate or the pulse width will often cause reductions on others of these characteristics, so that the net result is often an actual decrease in the etch rate. In addition, simply increasing the power delivered to the target material may cause undesired effects at the target material, such as loss of control of the drilling or cutting dimensions, ragged cuts, blocking of the laser beam by the ablated material, thereby actually reducing the power delivered to the material, and unwanted deposits of ablated material in the drilling or cutting area, thus damaging the surface of the target material.
As discussed above, UV DPSS lasers also have problems with beam pointing and drift stability because the laser rods are diode pumped, which causes the rods to heat and thereby results in problems with thermal drift and pointing stability. The thermal lensing and harmonic generation processes required to convert the fundamental IR radiation of the lasers to the UV range required for the desired laser output beam also creates further issues with stability. In addition, the optical efficiency of UV DPSS lasers is relatively low due to inefficient diode pumping methods and the resulting energy losses are translated into heat, which further aggravates the problems with thermal drift and pointing stability.
As also discussed above, UV DPSS lasers are limited to operating in either a pulsed mode output or a continuous mode output but a UV DPSS laser cannot generate both mode outputs simultaneously, thereby limiting the ability of a system using UV DPSS lasers to generate the output beamforms most suited for different applications and materials. UV DPSS lasers also suffer from beam propagation issues due to its beam propagation value which is approximately M2=<1.2, which creates problems when using beam shaping technology such as diffractive optical elements and holographic optical elements by causing ringing effects. The ringing effects cannot be compensated for and generate undesirable laser beam imaging properties at the target material being processed.
The methods and apparatus of the present invention provide solutions to these and other related problems of the prior art.